Developing a Disinfection Strategy

Oftentimes, people and companies are resistant to change. There are a lot of rituals, habits, practices and occurrences, and we have no idea how or why we do them. Whether it is social, work or family-related, there are times we may need to ask, “Why am I doing things this way?” Disinfection of drinking water is no different.

The most common way to disinfect drinking water has been by the use of chlorine in some form. However, over the years, newer technologies have been developed to disinfect water. Please note that the aim of this article is not to get you to change what you are doing, but merely to get you to see if what you are doing is actually the best situation for your facility. Sometimes familiarity can give a comfort level, and when this occurs, improvements, efficiency, environmental implications and effectiveness can be ignored. So a few relevant questions can be asked concerning the disinfection process that is used at the facility where you work.

  1. How effective is the current system?
  2. Is there a need to change disinfectants?
  3. How would an alternative disinfectant work in your system?
  4. Are primary and secondary disinfectants required?

If you know the answers to these questions, then you have evaluated your disinfection methods. If you do not, it may be time for you to get some answers. In either case, here are tips on how to evaluate your current situation. Primary disinfection is responsible for conventional treatment (microbial disinfection), and secondary disinfection is responsible for maintaining the residual throughout the distribution system.

There are three phases of disinfectant selection. The first step is to evaluate the current primary disinfectant, then select a primary disinfectant, and finally, select a secondary disinfectant. When evaluating the primary disinfectant, it should meet microbial inactivation limits for the following pathogens and conditions: Giardia Lamblia, Legionella, Heterotrophic Plate Count, total coliforms, turbidity and viruses. Also determine if inactivation can be met through process modifications, such as moving application points, increasing doses, increasing contact time or adjusting the pH values. Also make sure disinfection byproducts’ (DBP) levels are being met. See if the treatment process is optimized by checking the coagulation and filtration and studying results of moving disinfection points. When selecting the primary disinfectant, remember that the total organic carbon (TOC) value indicates the potential for disinfection byproducts. Unfiltered systems should consider using ozone to cut down on byproducts. The following charts can assist you in deciding which method works best for selecting a primary disinfectant.

Assimilable organic carbon is produced when strong oxidants are used as primary disinfectants when high levels of TOC (0.10 mg/L or greater) are present after filtration. If this is the case in your system, then additional treatment should be considered. Also remember that the potential of disinfection by product formation is expected if chlorine is used. When considering a secondary disinfectant keep in mind that disinfection byproduct content in the effluent should be limited. Distribution system retention time should also be considered. High retention time in distribution system is greater than 48 hours. Be aware of the pros and cons of retention and determine if booster stations will be necessary or not. If you choose chlorine dioxide as a secondary disinfectant make sure that high doses are not required as safety can become a concern.

The goal of the preceding information is for you to take some time and think about the level of efficiency, possible improvements, environmental implications and effectiveness of your facility’s current treatment process. If there are improvements to be made in any of these areas, this is the first step to strengthening the quality of your area’s drinking water, distribution system and environment.

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